Hybrid Cars: Parallel vs. Series
New variations on a very old idea

June 4, 2014

The hybrid car, powered by both an internal combustion engine (ICE) and a battery-driven electric motor, is not a new concept. The first hybrid was built in 1901 by Ferdinand Porsche, then a young engineer at the Lohner Carriage Works in Vienna, Austria. Porsche’s idea was to replace two tons of lead acid batteries in the company’s electric cars with a relatively lightweight gasoline engine. The gasoline engine drove a generator to recharge a smaller battery feeding electric motors in the wheel hubs. By 1905, the Belgian engineer Henri Pieper combined the motor and generator into a single unit and invented a single-lever control system that took care of all the drive and recharging functions, including regenerative braking. He got a U.S. patent on his system in 1909. In 1914, Hermann Lemp at General Electric figured out how to scale up the DC power controller to run a diesel-electric locomotive, and since then, hybrid drive has been common for railroad, ship and submarine propulsion.

These are all series hybrids, so called because the ICE drives the generator, which drives the electric motor, which drives the wheels. This arrangement works well in low- speed, high-torque applications because the electric motor produces maximum torque at zero revolutions per minute (rpm). Maximum cranking power is available to get the locomotive or the submarine moving, without the use of a complex, heavy clutch and transmission.

The series drive is less useful in high- speed cruise, because there’s an efficiency loss between the generator and motor— and a double efficiency loss if a battery sits in the electric flow between the generator and motor. In automobiles, that’s always the case. The driver may need more power, for quick acceleration or hill climbing, than the engine-generator combination can provide instantaneously — so you want some power in reserve, stored in the battery.

A car, however, is light and agile compared to a loco- motive. A sophisticated lightweight transmission can be designed to bypass the electric motor at cruising speed and connect the ICE directly to the wheels. The result is a parallel hybrid system, where the ICE and the electric motor work one at a time or in tandem, depending on what’s most efficient for the driving situation. Excess power from the ICE is routed through the generator to keep the battery charged and ready to pitch in for bursts of acceleration. This is the system that runs most of today’s hybrid and plug-in hybrid cars, including all the hybrids from Toyota, Honda, Ford, Hyundai and BYD.

The exception is the Chevy Volt. In principle it’s a simple series hybrid like Pieper’s 1905 car — the battery runs the electric motor to drive the wheels through a slick continuously-variable transmission. The ICE never drives the wheels but rides along until needed to recharge the battery. The Volt, to be sure, uses 21st century integrated-circuit controls to make smooth, quiet power throughout the speed range.

Any proper hybrid does better in stop-and-go driving than a pure ICE, because it doesn’t idle when stopped and because it recovers energy through regenerative braking. So do you want a series or a parallel hybrid? In theory, the series hybrid should be less complex and more easily adapt- able to a wide variety of auxiliary power plants, from diesels to Stirling heat engines. The Volt drive train will thus be the base for dozens of car and truck applications. But parallel hybrids are here right now and here to stay. So we’ll see.